The present invention relates to a powder of ytterbium phosphate of low infrared reflectivity having a non-stoichiometric phosphorus to ytterbium molar ratio or, more particularly, to a powder of ytterbium phosphate having an overstoichiometric phosphorus content relative to ytterbium which exhibits a remarkably low infrared reflection so as to be useful as an infrared-absorbing agent in an invisible ink for printing of an information data pattern, such as bar code patterns, optically detectable only by using an infrared detecting means but invisible to naked eyes or so-called stealth code patterns.
It may be too much to say that the bar code systems are very widely utilized in recent years in various fields including material distribution management systems. As is known, a bar code pattern is usually formed by printing a seemingly randomized stripe pattern on the surface of a substrate with a colored or, in most cases, black printing ink containing a black pigment such as carbon black. The bar code patterns formed by printing with such a black ink, though convenient in respect of good recognizability by naked eyes, on the other hand, have some problems due to the visibility of the pattern. One of the problems is that, because the bar code pattern occupies a substantial area on the substrate surface, decorativeness of the goods bearing the bar code pattern is greatly limited or decreased thereby not to allow a fully decorative design of, for example, packages as desired. The second of the problems is that, because the bar code pattern is readily and directly recognizable by any person handling the materials, secrecy of the information expressed by the bar code pattern cannot be secured sometimes.
In view of these problems, several attempts and proposals have been made, as disclosed in Japanese Patent Kokai 3-154187, 3-227378, 3-275389, 4-70349 and 5-93160, and elsewhere, to develop a printing ink invisible to naked eyes but detectable only by using an optical means such as an infrared detector.
One of the approaches in this regard proposed in these prior art documents is to utilize an infrared-absorbing organic dye of low infrared reflectivity including cyanine-based dyes, naphthoquinone dyes and the like. These organic dyes, however, are not quite satisfactory for the purpose because, if not to mention their relatively low fastness under exposure to ambience, the dyes are not completely colorless in the visible wavelength region of light but have, though weak, selective light reflection in the visible region so that patterns printed with an ink containing such an organic dye usually exhibit a slightly reddish creamy color not to give a complete solution for the above mentioned problems.
As a counterpart class of the organic infrared-absorbing materials, inorganic infrared-absorbing materials or inorganic materials having low infrared reflectivity but high reflectivity in the visible wavelength region are also under investigations. For example, Tajima in Japanese Patent Kokai 7-53946 proposes use of a powder of ytterbium phosphate as an infrared-absorbing agent useful as an ingredient in invisible printing inks. The ytterbium phosphate disclosed there is an orthophosphate of ytterbium, which is one of the rare earth elements having an atomic number of 70, expressed by the formula YbPO.sub.4. According to the disclosure there, the ytterbium phosphate powder was prepared by a process in which a blend of ytterbium oxide Yb.sub.2 O.sub.3 and phosphorus pentoxide P.sub.2 O.sub.5 was melted by heating at 1300.degree. C. for 2 hours in a platinum crucible and a solid obtained by annealing the melt, which was identified to be ytterbium orthophosphate by the X-ray diffractometric study, was pulverized into a powder. It is recommended that the rate of annealing is so adjusted that the crystallites forming the particles of the powder may have a diameter in the range from 5 to 200 nm in order to have high infrared absorptivity or low infrared reflectivity and the particles have an average particle diameter of 0.01 to 0.1 .mu.m with a maximum particle diameter not exceeding 1 .mu.m in order not to exceed the thickness of films formed by printing with a printing ink containing the powder. While ytterbium compounds in general, such as oxide and hydroxide, exhibit low infrared reflectivity, the infrared reflectivity of these compounds is not low enough as compared with the phosphate and Tajima teaches that the infrared reflectivity of the ytterbium phosphate powder can be controlled by adequately selecting the annealing rate of the melt.
The above described ytterbium phosphate powder disclosed by Tajima, however, is not industrially practicable not only because of the use of an expensive platinum crucible but also because of the use of a very high temperature of 1300.degree. C. or higher for melting of the blend of ytterbium oxide and phosphorus pentoxide necessitating special expensive furnace facilities in the preparation thereof.